Last time I asked around about this question, the answer was surprisingly “probably not much”! When a low-power x86 chip (like those mobile chips) is idling (which is pretty much all the time if all you are doing is hosting a server on it) it consumes very little power, about the same level as an idling Pi. It is when the frequency ramps up that performance-per-watt gets noticeably worse on x86.
Edit: My personal test showed that my x86 laptop fared slightly worse than my Pi 3 in idling power (~2 watts higher it seems), but that laptop is oooooooold.
It depends on your hardware if it is Debian. Debian prefers to use an older kernel until the next point release comes, which is nice because a random update likely won’t break your system.
But… speaking from experience: expect issues (missing drivers etc.) if your hardware is too new.